Tuning impedance for high radio frequencies



Dec. 1, 1942. G. E. PRAY TUNING IMPEDANCE FOR HIGH RADIO FREQUENCIESFiled Aug. 2, 1941 2 Sheets-Sheet l INVENTOI? ear e E H-a iagw v 1942- rG. E. PRAY r 2,303,338

TUNING IMPEDANCE FOR HIGH RADIO FREQUENCIES Filed Aug. 2, 1941 2Sheets-Sheet 2 as Q V U 24 l sell, 2 47 111:: l

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A TORNEY Patented Dec. 1 1942 UNITED STATES PATENT OFFICE TUNINGIIWPEDANCE FOR HIGH RADIO FREQUENCIES (Granted under the act of Marchv3,1883. as

amended April 30, 1928; 370 0. G. 757) 1 Claim.

This invention relates to novel impedance means for resonating at highradio frequencies.

In circuits tuned to very high radio frequencies the amount of inductivereactance required to resonate with the capacitance in the circuit is ofsuch a low value that a conventional type of wound coil inductance mustbe physically very small. This introduces serious practical diiiicultiesin constructing inductances of like values for the proper tracking oftuning condensers. Due to the low impedance of vacuum tubes at highfrequencies it is necessary that the connections between the tubes andthe tuned circuit be tapped down on the inductance in order to maintaina high resonant impedance in the circuit. Small coil inductances do notprovide a sufficient choice of tapping points to obtain satisfactoryoperation of the circuit.

One of the objects of the present invention is to provide an inductancehaving a low reactance for relatively large physical dimensions thusenabling it to be constructed and duplicated with precision. Anotherobject of the invention is to provide a means for progressivelyadjusting the tapping points along the inductance in small increments. Afurther object of the invention is to provide a group of high frequencytuned circuits which may be gang-tuned with accurate tracking. Anotherobject is to provide an inductance of confined field thereby reducingthe tendency toward stray coupling to other circuits. Still anotherobject is to provide a system of gang-tuned high Q circuits. Furtherobjects will become apparent from a study of the following descriptiontaken together with the accompanying drawings in which:

Fig. 1 is a diagrammatic showing of a typical preselector circuitemploying gang tuned segments of parallel transmission lines asimpedances;

Fig. 2 is a side elevation of a typical assembly of transmission linesegments of the type indicated in Fig. 1 showing the ganged tuningarrangement;

Fig. 3 is a diagrammatic showing of a typical preselector circuitemploying segments of concentric transmission lines as impedances;

Fig. 4 is a diagrammatic showing of a variation of the oscillatorcircuit portion of Fig. 3, together with its coupling impedance;

Fig. 5 is an elevational view in cross-section of a segment of atransmission line of the type indicated in Fig. 3 showing the use ofboth capacitive and inductive trimmers;

Fig. 6 is a plan View of a fragment of the transmission line of Fig. 5showing a detailed view of the capacitive trimmer, and

Fig. '7 is an elevational view in cross-section of a modification of thetransmission line segment shown in Fig. 5.

This invention makes use of segments of transmission lines as inductivereactances, one end of each line being loaded by capacitance for tuning,and the other end being short-circuited and grounded. In order for atransmission line to be inductive, it must be shorter in length than anyodd number of quarter waves for the frequency to which it is to betuned. For frequencies of the order of to 500 megacycles per second,lines may be designed to be shorter than one quarter wave length forconvenience. The resulting inductive reactance may then be tuned by theloading capacitance to any frequency within the limits of the design.One application for this invention is its use in the preselector circuitof a superheterodyne receiver.

In the circuit shown in Fig. 1 segments 8, 1 and 8 of paralleltransmission lines are utilized as coupling means between the antennaand the amplifier circuit 9, between that circuit and the mixer circuitl0, and between the latter circuit and the oscillator circuit II. Asmore clearly shown in Fig. 2, each of the transmission line segments 6,I and 8 comprises a pair of parallel conductances I2 which areshort-cirouited at their lower ends by the bar I 3 and grounded as shownat M in Fig. 1. At the upper end of the segment a trimmer condenser isprovided, the fixed plate l5 of which is carried by one of theconductors, the movable plate l6 being carried by the other. Thiscondenser is manually adjustable during the alignment of the receiver. Atuning condenser I1 is also connected across the upper ends of theconductors I 2 of each segment, these condensers being gang tuned by ashaft l8 which terminates in the tuning dial l9. Adjustable taps 20 areprovided for connecting the segments in their respective circuits. Thesetaps are movable along their respective conductors.

Referring to Fig. l, the received signal is conducted from the antennaalong the low impedance transmission line 2! to low impedance points onthe first tuned transmission line segment 6. This segment is coupled tothe control grid of a radio frequency amplifier tube 22 by means of agrid lead 23 tapped at a point on the segment whose impedancecorresponds to the input impedance of tube 22. The signal is amplifiedin this tube and coupled from its anode to the I had control grid ofmixer tube 24 by means of lead 25 which is tapped to a point on thesecond tuned segment 1, the impedance of which corresponds to thecombined impedance presented by the amplifier anode and the mixer grid.The oscillator tube 26 is shown as a triode but is not restricted tothis type. In order to obtain uniformly strong oscillations throughoutthe frequency range the segment 8 coupled to the oscillator circuit iselectrically balanced with one conductor coupled to the oscillator tubegrid through lead 21 and the other conductor coupled to the oscillatoranode through conductor 28. The coupling points are so adjusted as toprovide proper impedance matching between the segment 8 and the tubeelements. The cathode is at ground potential. Oscillator voltage iscoupled from segment 8 by lead 52 through a capacitance 29 to the mixercathode for heterodyning the received signal. This capacitance alsoprovides a by-pass path to ground for the signal, since the impedance atthe tapping point of the segment connected to the oscillator frame isvery low at the signal frequency. The difference frequency between theoscillator and signal frequencies is coupled from the mixer anode to thefirst I. F. transformer or other load circuit. It is desirable to placethe capacitance 30 between the mixer anode and ground to provide aby-pass for the signal and oscillator frequencies. This capacitancebecomes a portion of the I. F. resonant circuit at the intermediatefrequency, so introduces no losses.

For trimming the inductance of the oscillator circuit. an adjustableslidable bar 3|, is attached to the conductors of the transmission linesegment 8 associated with that circuit as shown in Fig. 2. This bar maybe moved along the conductors and acts as a short-circuiting element toeffectively shorten the length of the segment and produces acorresponding reduction in inductance.

the same relative positions in the circuit as the segments 6, 1 and 8respectively in Fig. l. The circuit of this figure is essentially thesame as that shown in Fig, 1 except that the oscillator 28 operates withits anode at radio frequenc ground,

its grid at high radio frequency potential and its cathode at anintermediate point. The resonant oscillator circuit is coupled to theoscillator tube grid, to its cathode and to the mixer cathode by meansof inductive coupling loops 50, i and 49 respectively, and inductivecoupling loop 48 is also substituted for the tapped connection of thetransmission line 2| to the amplifier segment. In both circuits it issometimes found desirable to provide radio frequency chokes 35 in themixer heater leads effective over the oscillator range in order toprevent the oscillator injection voltage from being by-passed to groundthrough the mixer cathode-heater capacitance.

As a variation of the inductively coupled oscillator unit of Fig. 3 aconductively coupled oscillator circuit is shown in Fig. 4, in which theoscillator anode operates at radio frequency ground by virture of theconnection of the lead 28 to the outside of the segment 34. The gridcircuit 21 is tapped directly at a high potential point 54 and theoscillator and mixer cathodes are tapped at low potential points such as55 and 59 on the inductive center conductor. The oscillator of Fig. 4may replace that of Fig. 3 by merely connecting the three fioating leadsin place of the three corresponding leads in Fig. 3. Fig. 5 shows thestructural detail of one of the concentric line transmission segmentsutilized in Fig. 3. A tuning capacitor 36 is connected between the outerand inner conductors, its movable plates being mounted on a shaft 3'!which also carries the movable plates of the corresponding capacitors ofthe other segments in the circuit. The arrangement for gang tuning isthe same as that illustrated in Fig. 2. A capacitive trimmer is providedand, as more clearly shown by Fig. 6, consists of a fixed plate 38carried by the inner conductor and a movable plate 39 carried by a screwthreaded shaft 46 which is carried in a threaded portion 4i attached toa bracket 42 mounted on the outer conductor.

An inductive trimming means is also provided for use in the segment 34which is coupled to the oscillator. This means is shown in Fig. 5 andconsists of a pair of electrically conductive plates supported onvertical rods 44 extending upward from the bottom of the segment. Theseplates in the magnetic field may be adjusted by rotating the rods M inwhich they are supported. For this purpose the rods extend through thease of the conductor in order that their ends may be engaged by a wrenchor other adjusting tool. The rods 44 may be rotated to position thetrimmers at any angle between parallelism with the magnetic field andperpendicularity thereto. A rotation of of the rod carries the trimmerthrough its maximum range, the greatest inductive effect being securedwhen the trimmer is perpendicular to the inductive field. It maysometimes be desirable to provide inductance trimming in the radiofrequency and mixer circuits, where extreme accuracy of alignment andtracking is required. However, it has been found that adjustment of thetapping points will normally provide sufilcient inductive trimming forthese circuits.

Fig. '7 illustrates another form of concentric transmission line segmentsimilar to that shown in Fig. 5, but having large openings such as thatindicated at 45 for the purpose of allowing access to its interior. Inall other respects this form of impedance is the same as that shown inFig. 5, although the trimmers have been omitted.

In a wound inductance, with one end of the coi1 grounded, the potentialon adjacent turns are in phase, and the mutual inductive efi'ectincreases the net inductance of the coil. This results in the electricallength of the coil being much greater than its physical length. The highdistributed capacity present in the coil tends further to increase itselectrical length. The physical dimensions of a coil being so much lessthan its electrical dimensions it becomes very difiicult and impracticalto accurately design and duplicate coils to obtain an inductance of lowvalue.

However, in a transmission line such as those of Fig. 1 the potentialsat corresponding points along the two parallel conductors are of equalmagnitude in opposite phase, tending to cancel any mutual effect andconfine the inductive field to the space immediately adjacent thetransmission line. In such a transmission line the distributed capacityis very low. If this line had no capacity loading at the end, itsphysical length would be nearly equal to its electrical length. Byshortening the line to the proper inductive value it may be tuned over awide frequency range by a small variable capacitance. Accurate designand duplication of such inductances is very simple and very practical.

In a concentric transmission line such as those used in the circuit ofFig. 3 and further illustrated in Figs. 5 and '7 the outer conductor isat a nearly uniform potential throughout its length, such smallvariations as may occur being of opposite phase from those along thecenter conductor. Thus the same reasoning as given above for paralleltransmission lines applies here also. The inductive field in this caseis confined entirely within the transmission line. The space betweenconductors should be small compared to one-quarter wave length for bothtypes of inductance. In designing a tuned segment of paralleltransmission line as illustrated in Figs. 1 and 2, its inductance may bedetermined from its dimensions or vice versa.

X Z tan 21% where X =inductive reactance= 2DfL ohms l=physical length(usually in centimeters) A=wavelength of operation in same units as l Z=surge impedance in ohms 276 log g b=spacing, center to center, ofparallel conductors a=radius of conductor.

The quantities b and a where found herein may be in any like units sinceonly their ratio is employed. However, for convenience they are usuallyexpressed in the same terms as Z.

In designing a concentric line segment, as shown in Figs, 3, 5 and '7,its inductance may also be determined from the above equation for XL,where where b=inner radius of outer conductor a=outer radius of innerconductor Another method for solving the short concentric line is thetoroid equation where L=inductance in microhenries l=physica1 length ininches.

In aligning the circuits, the normal procedure is to adjust thecapacitance trimmers near the high frequency end of the tuning range,and the inductive trimmers near the low end, repeating the process untilsatisfactory tracking is obtained.

It should be understood that the practice of the invention is notlimited to the embodiments illustrated and described but iscircumscribed only by the scope and limitations of the appended claim.

The invention described herein may be manufactured and/or used by or forthe Government of the United States of America for governmental purposeswithout the payment of any royalties thereon or therefor.

I claim:

A preselector circuit comprising an amplifier circuit, a mixer circuitand an oscillator circuit, a series of inductive transmission linesegments coupling said circuits together and coupling said amplifyingcircuit to an antenna, means conductively coupling and grounding theconductors of each of said segments at one end thereof, a capacitorcoupling said conductors at the other end of each segments, a capacitivetrimming means associated with each of said segments, movable tap meanson the conductors of each of said segments, whereby the points ofconnection of the leads of said circuits may be varied for the impedancetrimming of said segments, and an inductive trimming means associatedwith the one of said segments coupling said oscillator circuit with saidmixer circuit.

GEORGE E. PRAY.

